The present invention relates to an automatic steering system for non-railed transfer cranes using pneumatic tires, and more particularly to an automatic steering method and apparatus for a non-railed transfer crane, in which steering deviations occurring during the crane's travel are detected using a vision sensor and corresponding signals are used to automatically control the crane's steering so that the deviations can be corrected.
Generally, non-railed transfer cranes ("non-railed transfer crane" and "crane" are used interchangeably hereinafter) have been widely used for transferring containers or cargos, such cranes typically being equipped with pneumatic tires. The non-railed transfer crane travels along set lanes to transfer cargoes or to load/unload goods. However, non-railed transfer cranes tend to deviate slightly from the set lanes during travelling. That is, measurable deviations of distance and angle between the crane's actual travelling path and set lanes frequently occur. These deviations occurring during the crane's travels result from variations in cargo mass and from non-linearities in the driving motor of the crane.
In order to cause the crane to travel straight along its set lanes, deviations of angle and distance must be compensated in real time. Deviations heretofore have been controlled either manually by an operator, or automatically by an automatic steering system.
FIG. 1 shows a top view of a conventional crane 10. Wheels 12 equipped with pneumatic tires are attached at the four corners of the crane's main body 11, and the crane 10 transfers cargoes 14 travelling along lane 13. If the crane is operated manually, the inexperience of an operator can cause the wheels 12 to deviate from the lane 13. The use of automatic steering systems (i.e., machine controlled) solves many problems associated with manual steering systems.
One type of automatic steering system uses current-looping wire. In such an automatic steering system, wires are laid under the crane's lane of travel, a predetermined amount of electric current is supplied to the laid wire, and the magnetic field resulting from such current flow is sensed by the crane, inducing the crane's motion along the wire path. However, there are drawbacks to automatic steering systems using current-looping wire. First, the installation of the current looping wire which induces the crane's travelling is quite expensive. Second, damage to the wire resulting from subsidence of the ground where the wire is laid can occur. Third, reinforcing bars found in concrete slabs, and power wires installed in the crane itself can cause errors in the current sensed from the looping wire.
To overcome the above-identified problems, it has been proposed to use a gyro sensor as one of the automatic steering systems. A gyro sensor senses the angle of deviation, indicating the degree to which an off-centered wheel of the crane deviates during the crane's travel. That is, as shown in FIG. 2, the gyro sensor measures deviation angle (.theta.) between the crane's actual path of travel and lanes 13. An encoder is attached to the wheel in order to measure the travelling distance (l) of the crane. The deviation angle (.theta.) and travelling distance (l) obtained by the gyro sensor and encoder, respectively, are processed by a microprocessor to calculate the deviation distance (.delta.). The microprocessor then generates a control signal for controlling a driving motor of the crane so as to minimize the deviation distance (.delta.). Thus, the travelling distance (l) of the crane is compensated by the control signal, thereby preventing further deviation of the crane's wheels, and returning the crane to its intended direction.
Automatic steering systems with such a gyro sensor have the following disadvantages, however. First, deviation angle (.theta.) data sensed by the gyro sensor experiences a drift phenomenon which unstably varies the data with time and temperature. As shown in FIG. 3, the measured deviation angle (.theta.) does not remain constant over the time. The value changes slightly with variations time. If measured deviation angle (.theta.) is unstable, the value of deviation distance (.delta.) is incorrectly calculated, thereby causing errors in steering control. Second, when difference values of deviation distance (.delta.) based on the deviation angle sensed by gyro sensor are accumulated, errors of deviation distance obtained by the drift phenomenon are also accumulated. Therefore, the accumulated values of the error must be reset to a predetermined compensation value whenever the crane travels a certain distance, in order to prevent accumulating errors. Third, a setting device is additionally required for setting the deviation angle (.theta.) to "0" whenever the crane travels.